The present invention relates to an electropneumatic displacement device for a throttle valve of an internal combustion engine in idling condition. The device has a housing and a diaphragm arranged in a chamber in the housing, the diaphragm forming an outer wall of the chamber and of the housing and being operatively connected to the throttle valve. A spring in the chamber acts with one end against the diaphragm. The device also has a connection for atmospheric pressure, a magnet coil and an armature which is axially displaceable in the magnet coil and to which a valve body is firmly attached. The valve body has a connection for atmospheric pressure which is open or closed, corresponding to valve position at the time.
Such a displacement device has been described in Federal Republic of Germany AS 28 30 738. The diaphragm is arranged between two diaphragm plates against which a compression spring rests. Corresponding to the difference in pressure between the two chambers, the diaphragm arches toward the vacuum side, whereby a correcting element which acts on the throttle valve is correspondingly displaced. Due to the initial tension of the spring, a displacement of the diaphragm or of the correcting element by the connection of a load is prevented. In order to compensate also for changes due to the connection of a further load, a solenoid valve is provided which is actuated by the connection. Depending on the switch position of the solenoid valve, a connection for atmospheric pressure is opened or closed, as a result of which a vacuum in the one chamber can be rapidly eliminated.
In this known displacement device, however, no measure has been taken against the contingency that under certain circumstances the throttle valve may move only with difficulty whereby the setting force required for the actuation of the correcting member becomes larger. In this case, a larger setting current must be fed to the displacement device in order not only to overcome the resistance to the displacement but also, to effect a displacement. Without additional measures it may happen, due to the frictional hysteresis, that the throttle valve is not displaced at all. A displacement actuation of the displacement device which then takes place without any actual effect leads to so-called sawing, i.e. the setting signal given off by the controller has no effect on the correcting member and the control circuit regulates continuously around the desired value.